Field of the Invention
The present invention relates to a sheet stacking apparatus configured to stack sheets which are sequentially conveyed.
Description of the Related Art
In an image forming apparatus such as a copying machine, a printer, a facsimile machine, or a digital multifunction device, sheets having images formed thereon are sequentially delivered to a post-processing apparatus. The post-processing apparatus is configured to deliver the introduced sheets directly to a tray and stack the sheets on the tray, or perform various post-processing and stack the sheets on the tray.
Even when a sheet is not subjected to particular processing in the post-processing apparatus, application of heat to the sheet for the purpose of fixing toner in the image forming apparatus may cause the sheet to shrink unevenly, and particularly cause edges of the sheet to roll up and swell (in other words, the edges are curled). Further, steps of the post-processing may include, after aligning edges of conveyed sheets, stapling of saddle stitching, nipping the bound bundle of sheets with a pair of rollers to crease the bundle of sheets, delivering the creased bundle of sheets to a delivery tray, and stacking the bundle of sheets on the delivery tray. Then, when the center-folded sheets are sequentially delivered to the delivery tray, the sheets are introduced to the delivery tray with respective folded portions, which are bent portions, being the leading edges.
However, when the bundle of sheets is center-folded, the bundle of sheets may swell. In particular, such swelling may occur conspicuously at the folded edge of the bundle of sheets. Thus, there is a problem in that a stack height of the bundles of sheets readily exceeds a mechanical size limit of the delivery tray, and hence stacking a large number of the bundles of sheets is disabled, thereby degrading the stacking efficiency.
Thus, there has been known a sheet stacking apparatus having a configuration in which, when the bundles of sheets are to be stacked and received on the delivery tray, a receiving surface of the delivery tray configured to receive the bundles of sheets is inclined along a direction of introducing the bundles of sheets, and the stacked bundles of sheets are pressed by an arm (see Japanese Patent Application Laid-Open No. 2006-143466).
However, according to the related art disclosed in Japanese Patent Application Laid-Open No. 2006-143466, when an area of the receiving surface of the delivery tray configured to stack the sheets thereon is sufficiently larger than a surface area of the sheets, there is a case where sheets subsequently introduced to a portion of the delivery tray having no sheet stacked thereon are moved, while sliding on the inclined receiving surface, to a portion having sheets stacked thereon. In such a case, there is a problem in that the subsequent sheets sliding on the inclined receiving surface hit the sheets having already been stacked.